In oil wells, natural gas wells, and the like (hereinafter also collectively referred to as “oil wells”), oil country tubular goods are used for extraction of underground resources. Oil country tubular goods, which are steel pipes, are configured to be sequentially connected to each other, and threaded joints are used for the connection (see Japanese Patent Application Publication No. H09-119564 (Patent Literature 1), Japanese Patent Application Publication No. H10-89554 (Patent Literature 2), and Japanese Patent Application Publication No. H09-273671 (Patent Literature 3)).
Such threaded joints for steel pipes are generally classified into two types, a coupling type and an integral type. In the case of a coupling type, one of a pair of tubular goods that are to be connected to each other is a steel pipe, whereas the other thereof is a coupling. In this case, the steel pipe is provided with a male threaded portion formed on the outer circumference at each end thereof, and the coupling is provided with a female threaded portion formed on the inner circumference at each end thereof. The male threaded portion of the steel pipe is screwed into the female threaded portion of the coupling, thereby making up a joint and connecting them. In the case of an integral type, a pair of tubular goods that are to be connected to each other are both steel pipes, and no separate coupling is used. In this case, each steel pipe is provided with a male threaded portion formed on the outer circumference at one end thereof and a female threaded portion formed on the inner circumference at the other end thereof. The male threaded portion of one of the steel pipes is screwed into the female threaded portion of the other of the steel pipes, thereby making up a joint and connecting them.
In general, the joint portion at the pipe end portion where a male threaded portion is formed is referred to as a pin because it includes an element that is inserted into a female threaded portion. On the other hand, the joint portion at the pipe end portion where a female threaded portion is formed is referred to as a box because it includes an element that receives a male threaded portion. A pin and a box both have a tubular shape because they are constituted by end portions of tubular goods.
As threaded joints for steel pipes, threaded joints with tapered threads as specified by API (American Petroleum Institute) standards are generally used. Also, as disclosed for example in Patent Literature 1, threaded joints with modified API buttress threads (trapezoidal threads) and having improved torque resistance capability are used. The torque resistance capability refers to the ability to prevent deformation, fracture, or the like even when excessive torque is applied to threads in a made-up state.
The threaded portions of the threaded joint disclosed in Patent Literature 1 include tapered threads having a generally trapezoidal thread form. Of the two side surfaces of each thread of the threaded portions, the side surface located at the trailing side in the direction in which the male threaded portion is screwed during make-up is referred to as a load flank. The side surface located at the leading side in the direction in which the male threaded portion is screwed during make-up is referred to as a stabbing flank. The load flanks of Patent Literature 1 have an angle (load flank angle) with respect to a plane perpendicular to the thread axis (hereinafter also referred to as a “joint axis” or “pipe axis”) which is in the range of −20 degrees to less than 0 degrees. The load flank angle, if expressed in terms of an angle formed by the load flank with respect to the pipe axis in the screwing direction, is in the range of greater than 90 degrees to 110 degrees. The stabbing flanks of Patent Literature 1 have an angle (stabbing flank angle) with respect to a plane perpendicular to the pipe axis which is in the range of greater than 30 degrees to 60 degrees. The stabbing flank angle, if expressed in terms of an angle formed by the stabbing flank with respect to the pipe axis in the screwing direction, is in the range of greater than 120 degrees to 150 degrees.
The stabbing flanks of Patent Literature 1 are provided with an interference fit. In the threaded portions of Patent Literature 1, in a made-up state, the load flanks of the male threaded portion and those of the female threaded portion are in contact with each other, and the stabbing flanks of the two portions are in contact with each other. In this state, crests and roots of the two portions are not in contact with each other, and clearances are formed therebetween. Such a threaded joint of Patent Literature 1 has high torque resistance capability by virtue of a wedge effect produced by the contact between the load flanks and between the stabbing flanks.
The pin of Patent Literature 1 is provided with an abutment surface, called a shoulder surface, located at the leading end thereof. The box is provided with a shoulder surface that corresponds to the shoulder surface of the pin. In make-up of the pin with the box, the shoulder surface of the pin is abutted against the shoulder surface of the box, so that they are brought into contact with each other. Upon abutment of the shoulder surfaces, the tightening torque which is required for screwing of the pin increases rapidly. This phenomenon, in which shoulder surfaces are abutted against each other, is referred to as shouldering, and the tightening torque that is generated at the moment of shouldering is referred to as shouldering torque.
If the screwing of the pin is excessively performed after shouldering, yielding (plastic deformation) occurs in the regions of the shoulder surfaces, so that the tightening torque no longer increases or rapidly decreases. This phenomenon and the tightening torque that is generated at the moment when this phenomenon occurs are referred to as overtorque.
If make-up can be completed with a tightening torque that is in a range between shouldering and overtorque, the threaded joint will exhibit the best performance. That is, a suitable axial tightening force is generated within the threaded joint, so that the threaded portions are brought into firm engagement so as not to be loosened easily. In addition, in many cases, threaded joints are provided with seal portions which are mated together and placed in intimate contact around the entire circumference in a state in which the pin and the box have been made up. In this case, the seal portions form an interference fit therebetween as designed, as long as the make-up has been completed with a tightening torque in a range between the shouldering and the overtorque, and therefore desired sealing ability is achieved. Thus, when performing a make-up operation, a target tightening torque value is predetermined to assess the completion of the make-up. Such a target torque is set to a value between the shouldering torque and the overtorque.
If, for some reason, the tightening torque increases abnormally to greater than the target torque before shouldering actually occurs, the make-up operation will terminate in a state of insufficient tightening so to speak, and therefore a sufficient axial tightening force will not be introduced. In such a case, there is a possibility that the engagement of the threaded portions may become loose and thus a predetermined strength may not be obtained or that the contact force at the seal portions may be insufficient and thus a predetermined sealing ability may not be obtained. This phenomenon is referred to as high shouldering (a problem of shouldering torque becoming higher than a target torque).
On the other hand, if the tightening torque increases to greater than the overtorque, the state of excessive tightening so to speak will occur, which will cause plastic deformation in the regions of the shoulder surfaces. If this occurs, in the case where the seal portions are provided adjacent to the shoulder surfaces, the deformation of the regions of the shoulder surfaces is likely to lead to deformation of the seal portions and therefore formation of gaps therebetween, which results in a significant decrease in sealing ability.
As described above, the threaded joint of Patent Literature 1 is configured such that, in a made-up state, the load flanks of the threaded portions are in contact with each other and the stabbing flanks thereof are in contact with each other, and thus there are no clearances between the load flanks or between the stabbing flanks. Therefore, even when a high tensile load or compressive load is applied to the threaded joint in the axial direction, it is not likely that the pin and the box are relatively displaced in the axial direction. In particular, when a compressive load is applied to the threaded joint, the stabbing flanks of the threaded portions bear most of the compressive load, and therefore the deformation of the regions of the shoulder surfaces is inhibited and thus the deformation of the seal portions is also inhibited. As a result, the threaded joint of Patent Literature 1 has a sealing ability higher than that of threaded joints that employ API buttress threads.